We propose to study the mechanism of oxygenation of selected types of compounds by liver cytochrome P-450 oxygenase. It is felt that our present knowledge on this subject deserves considerable supplementation and clarification. From previous findings in our laboratories we concluded that an oxygen molecule ( 2) instead an atom (01) could enter into the substrate to yield, initially, hydroperoxidase or peroxide. Hydroperoxides and peroxides can react spontaneously with the genetic apparatus either by covalent binding or alteration which might initiate cancer. The oxygenation step appears to be radical in nature. The position specificity resides largely in the substrate molecule as opposed to the enzyme. Our aim is to show that the O2 mechanism is indeed operative with chemical carcinogenesis. We would like to extend our knowledge on the reactions of hydroperoxides and peroxides with genetic apparatus.